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Introduction

The Asteroidea is one of the largest and most familiar classes within the Phylum Echinodermata. These animals, commonly known as sea stars or starfishes, form a diverse and speciose group. There are approximately 1600 extant species (Hyman 1955; Clark 1977; Clark and Downey 1992) which are found throughout the world's oceans. Following the classification of Blake (1987), these species are grouped into seven orders: Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida.

Like other echinoderms, asteroids are important members of many marine benthic communities. They can be voracious predators, having significant impacts on community structure. For example, Paine (1966) used Pisaster ochraceus to illustrate his concept of the role keystone species play in community ecology. The crown-of-thorns starfish, Acanthaster planci, is particularly well-known because it can cause extreme detrimental effects to coral reefs, particularly during population outbreaks (Moran 1988).

The controversial Concentricycloidea (a proposed sixth class of the Echinodermata; Baker et al. 1986, Rowe et al. 1988, Pearse and Pearse 1994) have been diagnosed as unusual asteroids (Smith 1988, Belyaev 1990, Janies and Mooi 1999). Their relationship to other asteroid taxa is not well resolved, but alliances with species from the Velatida and the Forcipulatida have been proposed. The unique morphology of the concentricycloids makes it difficult to assign this group to the recognized asteroid orders and is cited as sufficient distinction for class recognition.

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Extant Orders of the Asteroidea

A survey of asteroid nomenclature arranged by order has been compiled. Clark (1989, 1993, 1996) and Clark and Mah (2001) list accepted names as well as synonyms, otherwise invalid names, references and ranges of type localities.

Brisingida—Brisingids are deep-sea dwelling asteroids. They usually have many (6-16) long, attenuated arms which are used in suspension feeding. The Brisingida contains about 100 species in 17 genera and 6 families. A preliminary phylogeny for this order has been produced by Mah (1998).

Forcipulatida—These asteroids are distinguished by their forcipulate pedicellariae, which are generally quite conspicuous on the body surface. The Forcipulatida contains about 300 species in 68 genera and 6 families. A preliminary phylogeny for this order has been produced by Mah (2000).

Notomyotida—These are deep-sea dwelling asteroids having flexible arms with characteristic longitudinal muscle bands along the inner dorsolateral surface. The Notomyotida contains about 75 species in 12 genera and 1 family.

Paxillosida—These asteroids are considered to be somewhat infaunal in that they can bury themselves partially under sandy sediments. They are characterized by some morphological features (e.g. pointed, unsuckered tubefeet) which have been considered primitive by some (see Discussion of Phylogenetic Relationships, below). The Paxillosida contains about 255 species in 46 genera and 5 families.

Spinulosida—These asteroids have a relatively delicate skeletal arrangement and completely lack pedicellariae. No fossil spinulosids have been found. The Spinulosida contains about 120 species in 9 genera and 1 family.

Valvatida—These asteroids are quite diverse, but are often characterized by their conspicuous marginal ossicles. Definition of this group has been the most variable and the ordinal definition of many families included here has been controversial (see Discussion of Phylogenetic Relationships, below). The Valvatida contains about 695 species in 165 genera and 14 families.

Velatida—These asteroids typically have thick bodies with large discs and interradial depressions. Contrary to Blake's (1987) classification, molecular evidence suggests a relationship between some velatid and valvatid families (see Discussion of Phylogenetic Relationships, below). The Velatida contains about 200 species in 25 genera and 5 families.

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Fossil Record

The earliest asteroids appeared in the Ordovician (Figure 4). However, at least two major faunal transitions have occurred within the Asteroidea concomitantly with large extinction events: in the Late Devonian (Blake and Glass in Webster et. al. 1999) and in the Late Permian (Blake 1987, Gale 1987, Blake et al. 2000, Blake and Elliot 2003, Blake and Hagdorn 2003). The asteroid orders as described here contain all extant and some extinct species which have a morphology distinct from Paleozoic forms (i.e. Ambuloasteroidea; see Characteristics, Blake 1982, 1987, 1988; Gale 1987, Blake and Elliott 2003, Blake and Hagdorn 2003). The asteroid orders are thought to have appeared and diversified very rapidly (within approximately 60 million years) during the Lower and early Middle Jurassic, frustrating our understanding of ordinal relationships (see discussion below).

Relationships among Paleozoic asteroids, as well as between Paleozoic asteroids and extant asteroids, are difficult if not impossible to determine because of the limitations of the asteroid fossil record. Asteroid fossils are rare because 1) the skeletal elements rapidly dissociate after death of the animals 2) asteroids typically have a large body cavity that collapses with deterioration of the organs, resulting in misshapen forms and 3) asteroids often live on hard substrates which are not conducive to fossil formation. From the limited fossil evidence that is available we know that the basic body plan of the asteroids has remained the same since the Ordovician. Several papers by Blake (e.g 1989, 2000) describe limitations of the fossil record in detail.

Despite the paucity of the asteroid fossil record, fossil evidence has aided our understanding of asteroid evolution within both the Paleozoic and post-Paleozoic groups. One unique fossil fauna is that from the Hunsrück Slate of Germany from the Lower Devonian. These Paleozoic forms are well preserved and show a variety of morphologies. The diversity exhibited in this faunal representation suggests that the diversity of life habits of Paleozoic asteroids was probably very similar to what we see today in modern species (Blake 2000). Fossil members of the post-Paleozoic fauna have also been found. The oldest known neoasteroid is the extinct Triassic genus Trichasteropsis (Blake and Hagdorn 2003, Figure 5). Blake (1987) recognized a new order, Trichasteropsida, to contain this taxon. The slightly younger Triassic genus Noriaster barberoi, diagnosed to the extant family Poraniidae (Valvatida), is the oldest-known fossil species belonging to a surviving family (Blake et al. 2000, Figure 5).

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Characteristics

Like other asterozoans, asteroids have a characteristic star-shaped body plan consisting of a central disc and multiple (typically 5) radiating arms. Asteroids are most easily distinguished from other asterozoans (the Ophiuroidea) by the structure of the arms. In asteroids, skeletal support for the arms is provided by the ossicles of the body wall, which merge with those of the central disc, giving the arm a very broad based attachment to the disc. This skeletal arrangement allows for the extension of a comparatively large coelomic cavity from the central disc into the arms, which serves to hold some of the animal's organ systems, namely the gonads and pyloric caeca. Additionally, this skeletal arrangement also limits lateral flexion of the arms. Locomotion by asteroids is accomplished almost exclusively by means of the podia (tubefeet) from the water vascular system. Differences in morphology between asteroids and ophiuroids are described further in Blake (1998) and Dean (1999).

Taxonomy of asteroids usually is based on externally observable characteristics of the skeleton, particularly the primary ossicular series which define the body wall (ambulacrals, adambulacrals, marginals, terminals, actinals, abactinals), as well as secondary ossicles such as spines, spinelets and pedicellariae. Works by Perrier (1884) and Sladen (1889) laid the taxonomic foundation of most asteroid groups. Many other authors have contributed to and/or refined the asteroid classification scheme, notably Fisher (1911, 1928), Verrill (1914), Fell (1963), Spencer and Wright (1966) and McKnight (1975). Blake and Elliot (2003) provide clear definition of ossicle terminology. Blake (1987) provides classification and diagnoses of asteroid groups.

Perhaps the most important ossicular series defining the Asteroidea is the ambulacral column, found along the oral surface of the disk and radiating arms and associated with two or four rows of podia. The asteroid ambulacrum is distinguished by erect ambulacral ossicles arranged in series along the length of the ambulacral column. Critical differences in structure and arrangement of the ossicles of the ambulacral column define two groups of asteroids: an extinct fauna restricted to the Paleozoic and the mostly extant (mostly) post-Paleozoic asteroids (Blake 1987, Gale 1987). Blake and Hagdorn (2003) recently recognized this distinction formally with diagnosis of a new subclass: Ambuloasteroidea, containing the Paleozoic Calliasterellidae and Compsasteridae in addition to post-Paleozoic asteroids (Infraclass Neoasteroidea Gale 1987).

Application of the extraxial-axial theory (EAT) to asteroid morphology significantly aids our understanding of ossicle homologies within the Asteroidea and between asteroids and other echinoderms (Mooi and David 2000, Blake and Elliot 2003, Blake and Hagdorn 2003). According to the EAT, the ambulacral and terminal ossicles of asteroids are axial elements. These ossicles are formed according to the Ocular Plate Rule (OPR) and are associated with the developing water vascular system during ontogeny as are the axial ossicles of other echinoderms. The remaining asteroid ossicle series are extraxial elements, which can be added during ontogeny without any particular ordering system (although secondarily ordered serial homologous elements are common in the asteroids, e.g. adambulacrals and marginals). In comparison to axial elements, extraxial ossicles are prone to much more evolutionary lability (Mooi and David 1997).

Synapomorphies of the crown group: Ambuloasteroidea

Summarized from Blake (1998; 2000), Mooi and David (2000) and Blake and Hagdorn (2003).

Deep ambulacral groove—The paired ambulacral ossicles are erect and arch across the arm axis forming a clearly defined furrow. The extent of the arch and definition of the furrow are expected to be weaker in the earliest asteroids, but these characters are difficult to observe in most fossil specimens.

Dorsal podial pores—The dorsal podia pores are passageways between ambulacral ossicles through which the tubefeet descend. These pores allow for internal protection of the ampullae, dorsal outpockets of the podia, which contract and expand with extension and retraction of the podia. The ampullae of earlier asteroids were external, in closed, cup-like podial basins formed by the ossicles of the ambulacral column.

Offset positioning of the ambulacral and adambulacral ossicles and differentiation of articulation structures in ossicles of the ambulacrum—These features describe a variety of related apomorphic characteristics of ambuloasteroids. Offset positioning of the ambulacral and adambulacral ossicles allows for soft tissue connections between the ambulacral and both adjacent adambulacrals which is further enhanced with differentiation of articulation structures on the ossicles. This arrangement allows more complex movement in the ambuloasteroids. In non-ambuloasteroids a single ambulacral ossicle abuts a single adambulacral.

Presence of an odontophore—The odontophore is a small interradial ossicle associated with the mouth angle ossicle. The odontophore is expected to the homologue of the axillary in Paleozoic asteroids.

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Physical Description

Morphology

Asteroids can range from less than 2 cm to over one m in diameter, although the majority are 12 to 24 cm. Arms extend from the body from a central disk and can be short or long. A majority have 5 arms, although some can have up to 40. Calcareous ossicles make up the internal skeleton.

The water vascular system of the sea stars open up at the madreporite, a perforated opening in the central part of the animal. Internally, the madreporite leads to a stone canal, made up of skeletal deposits. The stone canal is attached to a ring canal which leads to each of the five (or more) radial canals. Tiedemann’s bodies and polian vescicles are pouches on the ring canal whose function may be osmoregulation or hydraulic regulation within the water vascular system. Each radial canal ends in a terminal tube foot, which has a sensory function.

Each radial canal has a series of lateral canals that terminates at a tube foot. Each tube foot is made of an ampulla, podium, and usually a sucker.

The oral surface, under the central disc, is where the mouth is located. The hemal system parallels the water vascular system and probably distributes nutrients from the digestive tract. Hemal channels extend to the gonads.

Larvae are bilaterally symmetrical and adults are radially symmetrical.

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Trophic Strategy

Asteroids are mainly scavengers and carnivores. In many areas where they are found they are high level predators. Asteroids feed on slow moving prey, including gastropods, bivalves, barnacles, polychaetes and other invertebrates. They feed by grasping the prey, then everting their stomach and secreting primary enzymes on the prey. The digestive juices break down the tissue of the prey, which the asteroids then suck up.

Some asteroids are suspension feeders. Plankton and organic detritus sticks to mucus on the body surface and is moved by cilia to the mouth. A few species that use their pedicellariae to capture prey may even feed on fish.

Asteroids have a complete digestive system. The mouth leads to the cardiac stomach, which is what the sea star everts to digest its prey. The cardiac stomach leads to a pyloric stomach. Digestive glands, or pyloric ceca located in each arm. Enzymes are secreted through pyloric ducts. A short intestine follows the pyloric stomach and leads to the anus.

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Planktonic larval stages are probably the most vulnerable to predation. Calcareous ossicles probably discourage predation of the adults. Other predators include Hyperoodon ampullatus, the northern bottlenose whale. Asteroids can lose arms to predators and regenerate the arms later.

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Life History and Behavior

Behavior

The non-centralized nervous system allows echinoderms to sense their environment from all sides. Sensory cells on the epidermis sense light, contact, chemicals and water currents. Higher densities of sensory cells are found in the tube feet and along feeding canal margins.

Red pigmented eye spots are found on the end of each arm. These function as photoreceptors and are clusters of pigment-cup occelli.

Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones.

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Respiration

Starfishes are equipped with tube feet, and breath through structures known as papillae, tiny structures that are spread across the surface of the body. Oxygen from the water is absorbed by these structures, and by fluid in the main body cavity. Excretion of nitrogenous waste is also done through the papillae. A starfish's body fluid contains phagocytic cells known as coelomocytes, which surround waste material and forcefully eject it into the surrounding water.

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Life Cycle

Asteroids are deuterostomes. Fertilized eggs develop into bilaterally symmetrical planktonic larvae, which have 3-part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults. Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones. After settling, the larvae go through a sessile stage and metamorphose.

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Reproduction

Asteroids can regenerate arms and some can reproduce asexually as the central disc divides. In sexual reproduction, asteroids are mainly gonochoristic (having separate sexes), but a few are hermaphroditic. Asteroids usually have two gonads in each arm and a gonopore opening to the oral surface. Gonopores are usually at the base of each arm. Most asteroids are free spawners, releasing sperm and eggs into the water. A few hermaphroditic species brood their young. Spawning is probably nocturnal.

Although there is generally no parental investment beyond fertilization, a few hermaphroditic species brood their eggs. Brooding species are usually found in environments that are harsh for the larval stage.

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Evolution and Systematics

Evolution

Discussion of Phylogenetic Relationships

Phylogenetic hypothesis of the Asteroidea based on Blake (1987). These relationships, as well as intra-order relationships, are contentious and re-evaluation of asteroid phylogeny continues (see Discussion of Phylogenetic Relationships, below).

Specific use of phylogenetic methods in studies of asteroid evolutionary relationships began in the late 1980s. These analyses (using both morphological and molecular data) have resulted in conflicting hypotheses of asteroid phylogeny. Phylogenetic analyses are continuing to be re-evaluated with additional data. Since their results are still somewhat contentious, they have yet to initiate changes in our classification system.

Evidence from morphological characters

In 1987, two differing hypotheses of order level relationships were proposed based on analyses of morphological characteristics (Blake 1987, Gale 1987, Figure 6,7). These two phylogenies differ due to differences in opinion about character polarity (assigning ancestral or derived status to a particular state of a character) and the different morphological characters used in the analyses (note that Gale does not specifically use phylogenetic methods). Both authors emphasized the importance of ambulacral characters to asteroid classification and recognized the distinction between Paleozoic and post-Paleozoic forms (i.e. Ambuloasteroidea, Blake and Hagdorn 2003).

However, Gale (following McKnight 1975) focuses on the lack of suckered tubefeet in the Paxillosida, considering them primitive. As a result, his phylogeny reflects two major groups: a basal Paxillosida and the remaining asteroids, all having suckered tubefeet, which he termed superorder Surculifera. Blake considers suckered tubefeet to be the ancestral condition. His phylogeny reflects two major asteroid groups: superorder Forcipulatacea (Forcipulatida + Brisingida) and a clade of the superorders Valvatacea + Spinulosacea (Valvatida, Notomyotida, Paxillosida, Spinulosida and Velatida). Outgroup comparison with Calliasterella and inclusion of the Trichasteropsida results in a basal Forcipulatacea.

Figure 6. Blake's (1987) hypothesis of Asteroidea relationships.

Figure 7. Gale's (1987) hypothesis of Asteroidea relationships.

The differences in these proposed phylogenies highlight questions about asteroid relationships that are still unresolved. The identification of the basal (neo)asteroid group has been the driving question of additional studies (see Evidence from molecular characters, below). Additionally, ordinal definition, particularly for Blake's Valvatida, Velatida and Spinulosida, is problematic. Such problems were not new to Blake and Gale. Although many asteroid groups can be clearly defined morphologically (Forcipulatida, Brisingida, Notomyotida), asteroid morphology is complex and diverse. Other groups are less clearly defined.

Evidence from molecular characters

Additional phylogenetic analyses incorporating molecular data with morphological data (Lafay et al. 1995) and using molecular data alone (Wada et al. 1996; Knott and Wray 2000) were presented in an effort to resolve phylogenetic arguments. Unexpectedly, these studies have done little to elucidate asteroid relationships and may have only added to the confusion.

Lafay et al. (1995) present an unrooted phylogeny deduced from analysis of a combined morphological data set taken from Blake (1987) and Gale (1987) with unordered character states (Figure 8). Although very few taxa were studied, their phylogeny supports the definition of asteroid orders proposed by Blake but separates the Paxillosida from the Valvatida. Their analysis with molecular data alone (sequence data from 28S rRNA) results in several conflicting topologies due to weak phylogenetic signal. Most of this signal is masked by that from the morphological data set when the two data sets are combined. However, after evaluating several rooting positions using molecular data from other echinoderms in outgroup comparison, Lafay et al. (1995) conclude that the Paxillosida may not be monophyletic and that the paxillosid genus Astropecten may be the sister group to the remaining asteroids, reminiscent of Gale's phylogeny.

Wada et al. (1996) include more taxa for additional investigation of ordinal monophyly (Figure 9). In multiple analyses of their molecular data set (sequence data from 12S and 16S rDNA), they find that paxillosids are paraphyletic with the paxillosid genus Luidia as the basal asteroid taxon. In addition, the Valvatida is not monophyletic and a forcipulatid clade falls within a group of valvatids, a velatid and spinulosids, a relationship in stark contrast to that proposed by Blake (1987). Further, the Spinulosida are never grouped with the Velatida, which Blake (1987) proposed as their sister group and which previously were considered a group within the Spinulosida (Spencer and Wright 1966, McKnight 1975, Blake 1981a).

Knott and Wray (2000) expand taxon sampling even more, but their molecular data set (sequence data from mitochondrial tRNA and COI genes) fails to resolve questions of asteroid phylogeny (Figure 10). Significantly, the Paxillosida is not basal in their results (although Astropecten is not included). The results of different tree reconstruction methods are not in agreement, and basal groupings are only supported by bootstrapping in the Neighbor-Joining analysis. The proposed phylogeny is similar to Blake (1987) in that two lineages (one largely of forcipulatids and the other largely of valvatids) are recovered, but Valvatida and Velatida are not monophyletic and some velatids plus the Spinulosida fall in the forcipulatid clade.

Evidence that the Concentricycloidea are asteroids

The position of the Concentricycloidea has been contentious since its discovery in 1986. Close relationship between the Concentricycloidea and asterozoans is expected (see Baker et al. 1986, Rowe et al. 1988, Pearse and Pearse 1994, Mooi et al. 1998), but argument over its taxonomic position continues. As yet, no changes in taxonomy have been made. The morphological features of concentricycloids are so distinct (e.g. two circumoral canals, a single peripheral ring of podia) that the tendency to recognize them as a separate echinoderm class is quite strong. Pearse and Pearse (1994) included the Concentricycloidea in a phylogenetic analysis using morphological characters defined by Blake (1987) and found that Concentricycloidea fall outside the asteroid clade. Further clarification of skeletal homologies between concentricycloids and asteroids (Mooi et al. 1998) supports asterozoan affinities, but questioned placing concentricycloids as close relative to the asteroid order Caymanostellidae (Velatida; Rowe et al. 1988, Smith 1988, Belyaev 1990). Caymanostellids and concentricycloids have superficially similar body plans which may be due to convergence rather than true relationships (Pearse and Pearse 1994, Mooi et al. 1998). Contribution of DNA sequence data from Xyloplax turnerae and phylogenetic analysis of a combined morphological and molecular data set (Janies and Mooi 1999, Janies 2001), however, supports recognizing concentricyloids as asteroids. In these analyses, Xyloplax is in a clade with the forcipulatid Rathbunaster and not with velatids (although Caymanostellidae is not represented). Support for relationships within the Asteroidea is low, but Xyloplax is positioned well within the asteroid clade.

"Use 'foamy' materials in which any threatening crack will be in short order run into a hole. Not only does this reduce the chance of cracking, but it saves material--less can be more…The little hard bits of echinoderms, the ossicles, develop as single crystals, but they avoid the excessive brittleness typical of crystals by being especially holey, as in figure 16.9. Wood gains some material benefit from similar voids. Such materials come under the heading of 'cellular solids,' the term having no connection with 'cellular' in the strictly biological sense--but in the sense that Hooke…originally used the word for the microscopic holes in cork." (Vogel 2003:338-339)Learn more about this functional adaptation.

"Articulated strut (fig. 21.7). These share the common lattice of compression-resisting elements, but their joints (articulations) permit motion. We use them infrequently, but we do deliberately build joints into many bridges, for example, so the resulting mechanisms can distort safely under changing wind loads, varied 'live' or functional loads, or thermal size changes. Nature often uses the arrangement--major portions of vertebrate skeletons can be best viewed as mechanisms of articulated struts. The hard elements (ossicles) and their connections in echinoderms such as starfish provide another example.

Systems build around articulated struts combine nicely with muscles; sometimes, as in insect skeletons, the muscles are on the inside, but the principle is the same. Among the best features of these systems is their ability to alter shape or overall mechanical properties rapidly without having to change the properties of specific materials…But even tensile tissues other than muscle may sometimes change properties fairly quickly in response to some chemical signal. These alterations have been studied most extensively in the so-called catch connective tissue of echinoderms (Motokawa 1984; Wilkie 2002). A starfish undergoes an impressive mechanical transformation as it shifts from being limp enough to crawl with its tube feet on an irregular substratum to being stiff enough so the same tube feet have adequate anchorage when pulling open the shell of a clam." (Vogel 2003:438)Learn more about this functional adaptation.

Functional adaptation

"Starfishes, which have hard spiny skeletons and five (or more) arms or limbs in a star-like arrangement, are also adept at autotomy when caught by predators. Their subsequent regeneration, however, can be particularly dramatic. As long as the shed limb is not devoured by the predator and still contains a section of the central body disc of the starfish that shed it, this limb has the ability to regenerate into a complete starfish." (Shuker 2001:132)Learn more about this functional adaptation.